1. Field of the Invention
The present invention relates to an illumination optical system and, more particularly, to an illumination optical system including a reflecting mirror having a secondary curved surface such as an elliptical mirror.
2. Description of the Prior Art
In an illumination system using a discharge-type light source such as an ultra high-voltage mercury lamp, it is most effective to converge light by a secondary curved surface mirror such as an elliptical mirror or a spherical mirror. However, such a system is disadvantageous in that light rays are not present near the optical axis, providing an angle distribution wherein illuminating light rays are absent around the optical axis. When such a system is used for a microscope which forms an image of an object through an objective lens, light rays are absent at the center of the entrance pupil of the objective lens, providing an annular angle distribution. This means that peripheral light rays alone are used for an actual objective lens having slight aberration, which is not desirable. When the N.A. (numerical aperture) of the illuminating light is defined with respect to that of the objective lens, since the light rays are absent around the optical axis, the total amount of light obtainable is limited. When the N.A. of the illumination system is set such that the .sigma. value which is a ratio of the N.A. of the illumination system (condenser lens) to that of the objective lens and is given by: EQU .sigma.=(N.A. of illumination system)/(N.A. of objective lens)
is slightly smaller than unity, optimum focal depth and resolution are obtained. Accordingly, in an illumination system of this type, in order to limit the N.A. of the illumination system as needed, the illuminating light rays must be shielded by an aperture stop at the periphery of the system where the light intensity is highest. This results in the fact that the effective illuminating light is significantly decreased. It is possible to use a condenser lens which has a short focal distance which matches the preset .sigma. value. However, an actual light source is not a point source and has a physical size. For this reason, when the focal distance is shortened, light rays which are to be collimated through the condenser lens form a predetermined angle with respect to the optical axis. The light rays are then partially shielded by the stop, resulting in light loss.